1. Field of the Invention
This invention relates to a method for detecting in a vessel the surface level of a bed of polymer particles, such as polyethylene pellets.
2. Description of the Prior Art
Although this invention is described hereinafter, for sake of clarity and brevity, in respect of polyethylene particles, it is applicable to polymer particles that have similar dielectric constant values.
Heretofore, polyethylene has been formed by polymerizing ethylene while dissolved in a solvent such as hexane. The resulting single liquid phase solvent solution (solution or single phase solution) also contains a polymerization catalyst. The polymerization reaction is carried out in the single liquid phase containing at least ethylene and catalyst dissolved in a solvent. Optionally, one or more co-monomers can be present. For sake of clarity and brevity this invention will be described in respect of polymerizing ethylene alone to form linear high density polyethylene (HDPE). However, one or more co-monomers can be employed in the practice of this invention.
The polymerization of the ethylene monomer is carried out using a series of stirred reactors followed by a tubular (plug flow) reactor. The plug flow reactor is employed upstream of an adsorber to accomplish product uniformity with a uniform residence time distribution for the reactants in that reactor. This gives all portions of that process fluid essentially uniform residence time in the reactor.
Downstream of the last (plug flow) reactor a catalyst deactivator is injected into the solution, and the resulting mixture introduced into an adsorption pressure vessel which adsorbs various compounds and decomposition components from the single phase solution. The polymerization reaction is carried out at an elevated temperature of from about 150 to about 280 degrees Centigrade (C) at a pressure of from about 2,000 to about 4,000 psig. The adsorption step of this process is also carried out in this high pressure range.
The adsorbent material used in this pressure vessel is typically a particulate material. These particles adsorb from the single phase liquid solution catalyst, various catalyst moieties, and by-products (residue) from the decomposition of the catalyst deactivator. The adsorbent is typically activated alumina particles such as alumina spheres about 1.7 millimeters in diameter.
The HDPE process must be carried out in a single phase solution. If two phases (a polymer rich phase and a separate solution rich phase) is allowed to form in the reaction zone or in the adsorption zone, a phenomenon known in the art as “frosting” or “two-phasing” occurs wherein solid polymer forms and separates out from the single phase solution. Although two-phasing is desired downstream of the reactors and adsorbers, it is not desirable in the interior of the reactors and adsorbers because solid polymer that comes out of solution and deposits in the equipment in those zones.
Process conditions such as temperature, pressure, and mass composition of the single phase solution stream can determine whether the stream will stay in the single phase or move toward two-phasing. For example, an elevated ethane content can induce two-phasing. If two-phasing is allowed to continue unchecked, the reactors, adsorbers, and/or associated equipment in which the two-phasing occurs will eventually plug up with solid polyethylene deposits thereby requiring shutdown of the plant, and cleanup of at least the affected equipment, a costly event in terms of lost production and clean-up expenses.
Downstream of the adsorption step two-phasing is deliberately induced in a separation unit. This is accomplished by way of a series of de-pressurization steps to form the desired, at this point in the process, formation of distinct polymer rich and solution rich phases. In this separation step, the polymer rich phase is physically separated from the solvent rich phase.
The separated solvent rich phase is processed to remove impurities and to separate solvent from un-reacted ethylene so that the recovered, separate solvent and un-reacted ethylene streams can be recycled to and reused in the afore described ethylene polymerization process.
The separated molten polymer rich phase is processed in an extruding/melt cutting unit to convert the molten polyethylene to a solid extrudate that is cut to form the desired solid polyethylene pellet product of the process. The solid polyethylene pellets resulting from this process are right cylindrical in shape and are about ⅛ inch in diameter and length. These pellets (particles) are surface wet with water and/or solvent.
The polyethylene pellets are then processed in a de-lumping unit to remove some water and any oversized lumps of pellets. The polyethylene pellets recovered from this unit are passed to a feed tank which holds pellet feed for a stripper unit downstream thereof.
The downstream stripper unit, known in the art as the “pellet stripper,” holds in a large vessel a bed of polyethylene pellets that are surface wet with water and solvent. The pellet stripper operates to remove from that bed, by way of a counter current low pressure steam treatment, residual solvent from the polyethylene pellets residing in that vessel. A slurry of water and polymer pellets is introduced into an upper portion of the vessel. This slurry first encounters near the top of the vessel a conical strainer (screen). Water passes through the strainer and is removed from the vessel. The liquid wet polymer particles roll down the upper, downwardly inclined surface of the strainer and into the interior of the vessel to form a bed of pellets in the interior of the vessel.
After solvent stripping in the pellet stripper, the water wet polyethylene pellets are removed from the pellet stripper vessel and subjected to de-watering, drying, blending, storing, and shipping operations.
It is desirable to be able to reliably, accurately, and continuously determine the upper level of the bed of polyethylene pellets residing in the aforesaid pellet stripper vessel. This is important to the reliability of the polymer production process as a whole.
Heretofore, the level of the bed of polyethylene pellets in the pellet stripper vessel below the conical strainer was determined by what is known in the art as a Bindicator which employs a weight on a cable. The weight is lowered into the upper portion of the pellet stripper vessel until a change in tension is detected in the cable presumably by contact of the weight with the upper level of the bed of polymer particles, and that is taken as the upper bed level measurement point.
Bindicator devices were set to operate periodically and software was employed in an attempt to make the measurements represent a continuous signal since the bed level is a controlled variable in the overall process.
The Bindicator approach proved to be less than desirable from a reliability point of view. Numerous problems were experienced such as cable kinks, motor failures, lost weights resulting in downstream equipment damage, and the like.
The detection of the upper level of the polymer bed in the pellet stripper vessel by way of lasers or ultrasound equipment was determined not to be feasible because the steam environment in the vessel and the loose pellets rolling off the strainer interfered with measurements using those classes of technology.
The detection of the upper level of the polymer bed in the pellet stripper vessel by way of guided wave radar was initially thought not to be useful not only due to the falling pellets, but, more importantly, the low dielectric constant of polymers, particularly polyethylene, even more particularly high density polyethylene.
To determine the upper level of the bed of polymer particles, a guided radar pulse must reflect off of that upper level with a strong enough reflection pulse to be detected by the radar receiver. Generally, the higher the dielectric constant value of the reflection surface, the stronger the reflection pulse back to the receiver. Guided wave radar transmitting/receiving units in general have a dielectric value limit of 2.0 to 2.5 as a lower operating range whereas, for example, the dielectric value range of high density polyethylene is from about 1.6 to about 1.8.
It was thought that the combination of falling polymer particles and low dielectric constant values also made guided wave radar technology inapplicable.
However, pursuant to this invention, a combination of factors has been discovered that can make guided wave radar technology useful in the detection of the surface level of a bed of polymer particles.